Hybrid resistance in irradiated H2 heterozygous F1 hybrid mice to grafts of H2 homozygous parental strain marrow grafts defies the laws of transplantation genetics and is mediated by natural killer (NK) cells rather than T lymphocytes. NK cells express Ly49 receptors that receive negative signals from MHC class I antigens, e.g. Ly49C or I - H2-Kb, Ly49A or G2 - H2-Dd. There are also Ly49 receptors lacking an inhibitory motif in the cytoplasmic domain which are co-expressed with DAP12 proteins containing an activating motif, e.g. Ly49D, H, and U. Host defense functions of NK cells are largely regulated by these receptors that are expressed on various fractions of NK cells. The Ly49 receptors are very polymorphic and determine the genetic ability of mice to reject marrow grafts, to kill tumor cells and to resist virus and other infections. Based on our recent studies, we have three specific aims. Aim 1: Test the hypothesis that NK cells that express Ly49A/G2 receptors not only fail to kill Dd+ target cells, but also act on Ly49D+ NK cells lacking Ly49A/G2 to inhibit their function or viability. The approach will be to activate NK cells of B6.Ly49A transgenic mice and test their ability of irradiate B6 hosts to reject H2d marrow grafts. Aim 2: Test the hypothesis that certain strains of mice that fail to reject H2d marrow grafts lack Ly49D-like receptors or express Ly49G2 or A on all of their NK cells. The approach will be to transduce stem cells of FVB mice with a construct expressing both Ly49D and green fluorescent protein (GFP). These cells will be transferred to irradiate FVB mice to generate marrow cell chimeras, detected by flow cytometry for GFP. These mice will be challenged with H2d marrow grafts to determine if they can now reject. F(ab')2 monoclonal antibody fragments to Ly49G2 and A will be given to block negative signals. Aim 3: Test the hypothesis that activating receptors, e.g. Ly49H, mediate the genetic resistance of mice to Friend leukemia virus (FV) and murine cytomegalovirus (MCMV) infections, while inhibitory receptors are responsible for poor resistance to infections. Approaches include using mice transgenic for inhibitory receptors, e.g. B6.Ly49C or A, for testing viral resistance, and transferring Ly49H genes to susceptible mice by means of marrow cell transfers, as in Aim 2. These studies will amplify knowledge of NK cell immunobiology.